The present invention provides new and useful concepts in a surface height measurement system and method, also referred to as an autofocus (AF) system and method that would be used with an optical imaging system and method that images a substrate (e.g. in the production of a semi conductor wafer).
A current, known autofocus system and method involves imaging an array of slits onto the surface under investigation (e.g. a semiconductor wafer) at a grazing angle of incidence. The imaging beams are largely reflected from the substrate and the reflected image of the slit array is relayed to a receiving slit array. The light transmitted through each slit in the receiving array is relayed to a detector element—one for each slit. When the substrate moves up or down, the slit image translates relative to the receiving slit reducing the power on the detector element. To enable sensing of the direction of substrate travel, a vibrating mirror is placed in the pupil of the imaging system (sending side in current AF system). This vibrates the image of the sending slits on the receiving slits and the resulting signal can be decomposed into its first and second harmonics. The ratio of the amplitudes of the first and second harmonics is approximately proportional to the substrate z-position and is known as the “PSD signal”.
A basic aspect of the present invention provides a solution that uses largely the same imaging scheme, but uses an alternative to the PSD signal. The present invention has several advantages over the PSD signal in that the present invention does not require a moving element (other than the substrate under investigation), has a larger linear dynamic range, and is inherently less sensitive to patterns that may be present on the substrate.